Multi-port optical switching devices generally have multiple inputs and outputs, from and to which optical signals can be switched. Wavelength selective routers or wavelength selective switches (WSS) are typical examples of such devices. A number of WSS devices have been described in the prior art. In co-pending U.S. patent application, Ser. No. 12/066,249, having common inventors with the current application, and herewith incorporated by reference in its entirely, there is described a WSS in which a signal on one or more of several input ports is switched to a common output port, or vice versa. In common with other WSS designs, the switching process takes place in free space propagation of the manipulated light signals.
The switch structure described in application Ser. No. 12/066,249 utilizes conversion, preferably by the use of birefringent crystals, of optical signals input to any port of the switch, to light beams having a defined polarization, preferably linear, and which are mutually disposed in a predetermined plane with respect to the system plane in which optical manipulation of the beam traversing the WSS is to be performed. The beams are spatially wavelength-dispersed preferably by means of a diffraction grating. The light is then directed through a polarization rotation device, preferably a liquid crystal (LC) cell pixelated along the wavelength dispersive direction, such that each pixel operates on a separate wavelength. When the appropriate control voltage is applied to a pixel, the polarization of the light signal passing through that pixel is rotated, thereby blocking, transmitting or attenuating the particular wavelength channel passing through that pixel.
After beam polarization rotation, the light passing through each pixel is angularly deflected using a reflective beam steering element. The beam steering element is pixelated along the wavelength dispersive direction, such that each beam steering pixel also operates on a separate wavelength. When the appropriate control voltage is applied to a beam steering pixel, the wavelength component associated with that liquid crystal pixel is steered by the beam steering pixel towards its desired direction. The beam steering element can be any miniature element which is capable of controlling the direction of reflection of beams impinging thereon. Typical WSS's can use either an MEMS array, or an LCOS array, or a stack of serially disposed liquid crystal arrays and prismatic polarization separators disposed in front of a reflecting surface.
The steering of the beam through each individual pixel enables light of different wavelengths, after being transmitted or attenuated, to be directed to different output ports, according to the various paths defined by the beam steering angles. Additionally, the light of a specific wavelength can be blocked, in which case the beam steering is unused.
A typical multi-input to single output switch configuration, shown schematically in FIG. 1 as a “black box” WSS 12, has several input signal ports, 1 to 6 in the example shown, each signal being input through a collimator 11, which could be part of a collimator array, and a single output port 16. The WSS 12 is controlled to switch the pixels of the polarization rotation device 13 and of the pixelized beam steering device 14, shown in the embodiment of FIG. 1 as a MEMS mirror array, such that any one of these six input signals can be directed to the output port 16 as required. For clarity, the angular geometry of the WSS in FIG. 1 has been exaggerated in order to illustrate the different angles of incidence on the beam steering device from the different ports. However, for any switching configuration designed to transmit a specific input port to the output, the MEMS reflector element orientation may be such that another input signal is reflected to output from the device at another input port. Thus for instance, in the example shown in FIG. 1, while the input signal at port 1 is being switched by reflection in the appropriately aligned MEMS mirror pixel, to the output port 16, the angle of that particular MEMS mirror pixel may be such that an input signal at port 2 may be switched to input port 4. Such an output appearing at input port 4 would be an undesirable, spurious output, and a method is needed for preventing it. Other spurious output signals may occur while each beam is being steered by the beam steering device, and may be inadvertently coupled to another of the input ports as it sweeps across the switch, and this would generate an undesirable transient spurious signal in that port. In the above-referenced application Ser. No. 12/066,249, methods are described for generating a hit-less switching configuration, by adjusting the transmissivity of the polarization rotation device in the optical paths leading to undesired ports, such that output to them is blocked as the steered beam passes over them. However, this method is not applicable for preventing the previously mentioned spurious outputs arising from non-transient reflections, i.e. arising from legitimate back reflections from an unselected input port to an unselected output port.
A method, commonly used in prior art switches, of avoiding all such spuriously directed signals, and especially back reflected signals from unwanted paths, is to insert an isolator 18 at every input port, such that a signal can be input to a particular input port, but signals cannot be output therefrom. However, this involves the use of an additional component for every input port, this involving additional cost and in particular, additional space in the WSS module. Furthermore, conventional discrete isolators can be polarization dependent, such that the transmission of the device may vary with the orientation of the input polarization.
There therefore exists a need for a new optical, multi-pole, multi-way wavelength selective switch structure having a simple optical structure, and operative without the need for external isolators on each of the input ports, and having polarization independence.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.